For preparing semiconductor thin films, various film-forming processes have been employed in accordance with desired electrical or physical properties and in view of application uses.
For instance, there have been tried a plasma CVD process, reactive sputtering process, ion plating process, light CVD process, thermal CVD process, MOCVD process, MBE process, etc. Several of the above mentioned processes have now been employed and industrialized as optimum to forming semiconductor devices.
Among all, production apparatus using the plasma CVD process have been employed most generally and, for instance, amorphous silicon solar cells of large area have been produced mainly by a multi-separated chambers system (Japanese Patent Laid-Open Sho 59-61078) or a roll-to-roll system (U.S. Pat. No. 4,485,125 and Japanese Patent Publication Sho 62-43554). However, the photoelectric conversion efficiency of the thus produced solar cells is considerably lower as compared with that of an amorphous silicon solar cell of small area prepared in a laboratory. The reason is attributable, in the multi-separated chambers system, to that stability and homogenity of plasmas can no more be maintained upon forming deposited films along with increasing the area of RF discharge electrode and that an increase in the size of a film-forming chamber for forming the deposited film makes it difficult to remove impurities from the circumferential walls of the film-forming chamber, etc. and thus increasing the amount of the impurities incorporated into the deposited films. In the roll-to-roll system, although the reduction in the photoelectric conversion efficiency is rather improved as compared with that in the multi-separated chambers system, there is still a problem caused by the sliding movement of a substrate, that the plasmas lack in stability or a special device is required for the mutual diffusion of gases between each of the film-forming chambers.
Further, from a view point of production efficiency, there is a limit for the size of the discharge electrode in order to maintain the stability and the homogenity of the plasma in the multi-separated chambers system and there is also a restriction for the area of a solar cell that can be produced per one batch. Further, since it is necessary for evacuation of starting material gas, equalization of pressure, control for the substrate temperature, stabilization for the amount of starting gas introduced and matching control accompanying the initiation of the electric discharge, repeatingly, on every time between film-forming chambers in adjacent with each other upon transportation of the substrate, which makes it difficult to reduce the production cost and requires much step control also in maintaining the reproducibility.
Further, in the roll-to-roll system, since it is necessary for leaking the inside of all the film-forming chambers to atmospheric air, cleaning each of the film-forming chambers, replacing rolls, on every completion of the film-formation per one roll, it requires much time for the maintenance of the apparatus. In addition, since the film-forming chamber is exposed to the atmospheric air on every step, it requires a long time for setting up the chamber to a sufficient degree of vacuum and much care has to be taken for administration of reproducibility of the vacuum state (for example, degree of vacuum attained).
Furthermore, although RF power is used as the plasma-generating energy, in each of the foregoing production apparatus, it can not always be said that the electrical or physical properties of the deposited films formed by such plasma CVD process are satisfactory for forming semiconductor devices. Further, plasma stability or reproducibility during formation of the deposited film over a long period of time may occasionally become poor and this sometimes leads to one of major factors for remarkably reducing the production yield.
As means for overcoming such problems, Japanese Patent Laid-Open Sho 60-41047, for example, discloses a method of improving the productivity of group IV semiconductor films at high quality by increasing the film deposition rate upon forming the films by means of HR-CVD process (Hydrogen Radical Assisted CVD process).
Further, Japanese Patent Laid-Open Sho 55-141729 and Sho 57-133636, etc. have proposed a method of disposing electromagnets at the pheriphery of a cavity resonator and establishing conditions for ECR (Electron Cyclotron Resonance) as means for efficiently forming high density plasmas by using a microwave of about 2.45 GHz. Further, it has been reported in academic meetings, etc. that various kinds of semiconductor thin films have been prepared by utilizing the high density plasmas. And microwave plasma CVD apparatus of this kind have already been commercially available at present.
Incidently, in the HR-CVD process described above, hydrogen atoms in the excited state (hydrogen radicals) play an important role for the control in the film property and homogenity regarding the formation of a deposited film. However, there has not yet been made a sufficient study but left a room for the improvement on the method of producing the hydrogen atoms in the excited state in a great amount and homogenously and controlling the excited state thereof upon forming the deposited film, thereby controlling the chemical reactions upon forming the deposited film so that the property of a deposited film to be obtained be controlled optionally and stably.
On the other hand, in the microwave plasma CVD apparatus utilizing ECR, it is necessary to maintain the inner pressure of the plasma-forming chamber at less than about 10.sup.-3 Torr for establishing the ECR conditions, which restricts the inner pressure upon forming a deposited film. Further, under such a level of pressure, the mean free path of gas molecules becomes longer (up to 1 m) and because of this, the starting material gas for forming the deposited film diffuses near the microwave introducing window and is decomposed to thereby adhere deposits on the surface of the microwave introducing window or the cavity resonator, whereby making electric discharge instable or causing contamination of foreign matters into a deposited film formed on the substrate due to defoliation and scattering of the adhered deposits. Further, plasmas generated in the plasma forming chamber diverge along the diverging magnetic field of electromagnets disposed to the inside of the film-forming chamber and, thus, the substrate is exposed to the plasmas at a relatively high density. Accordingly, the deposited film easily undergo damages such as of charged particles to impose a limit on the improvement of the quality. In the step of stacking a plurality of deposited films at the time of preparing a semiconductor device, the interface property is likely to reduce due to the damages caused by the charged particles, etc., failing to improve the properties of the semiconductor device obtained.